Typically, the performance of an electronic system is limited by the quantity of noise exhibited by its components. For example, the performance of a wireless telecommunications terminal is typically limited by the quantity of phase noise produced by its oscillator.
Although a crystal oscillator typically exhibits little phase noise, it cannot be easily fabricated as a component of an integrated circuit. In contrast, an oscillator that is fabricated from semiconductor devices in an integrated circuit typically exhibits more phase noise than does a crystal oscillator.
Although there are several types of noise exhibited by a semiconductor device (e.g., thermal noise, shot noise, flicker noise, etc.), empirically, the largest contributing factor to phase noise in a circuit is flicker noise, which is also known as "1/f" noise because the output current noise spectral density of flicker noise varies inversely with frequency.
Empirically, the quantity of flicker noise exhibited by a device is affected by the process used to fabricate the device. Therefore, a semiconductor manufacturer can affect the quantity of flicker noise exhibited by the devices it fabricates by carefully measuring the flicker noise exhibited by the devices emanating from a fabrication line and by modifying the fabrication line process in response to the measured results. There are several techniques in the prior art for measuring, or attempting to measure, the flicker noise exhibited by a device.
FIG. 1 depicts a schematic diagram of an elementary technique in the prior art for measuring the flicker noise exhibited by a device, which in FIG. 1 is a MOSFET. In accordance with this technique an A/D converter directly measures the voltage, V(t), across two terminals of the MOSFET which has been biased with a steady DC voltage. The output of the A/D converter is then fed into a spectrum analyzer, which computes the output voltage noise spectral density, S.sub.VL, for the MOSFET based on the temporal variations in V(t). Typically, this technique does not yield a satisfactory result because the noise within the A/D converter usually exceeds, and therefore masks, the flicker noise in the device under test.
FIG. 2 depicts a schematic diagram of a second technique in the prior art for measuring the flicker noise exhibited by a device. In accordance with this technique, a low-noise amplifier in interposed between the device under test and the A/D converter to amplify the flicker noise with respect to the noise in the A/D converter. Typically, this approach does not yield a satisfactory result when the device under test is a semiconductor device. The reason is that the noise figure of the amplifier depends on matching the impedance of the device under test to the input impedance of the amplifier. This is particularly difficult to accomplish when the device under test has a variable, non-linear impedance as do semiconductor devices.
FIG. 3 depicts a schematic diagram of a third technique in the prior art for measuring flicker noise. In accordance with this technique, the noise exhibited by the device under test is measured indirectly by measuring the output voltage noise spectral density, S.sub.VL, in a low-noise load resistor, R.sub.L, that is in series with one terminal of the device under test. Because the load resistor and at least one terminal of the device under test are in series, the output current noise spectral density, S.sub.ia, manifested by the device under test is observable in the voltage fluctuations across the load resistor, assuming that the load resistor exhibits substantially less noise than does the device under test. In accordance with the third technique, the output current noise spectral density, S.sub.ia, is based on ##EQU1##
Preferably, the load resistor is a low-noise resistor, such as a wire-wound resistor, which has an impedance that is fixed and matched to the input impedance of the amplifier. When the DC bias and V.sub.IN are provided by batteries, this technique typically provides satisfactory results for low cut-off frequency MOSFETs. For the purposes of this specification, the "cut-off frequency" of a device is the highest frequency of operation at which the gain of the device equals or exceeds one.
Empirically, this technique usually does not provide a satisfactory measurement of the flicker noise in high cut-off frequency MOSFETs or in bipolar devices. Furthermore, because the technique requires batteries to power and bias the device under test, the technique is generally not amenable for use in an industrial environment such as a fabrication line.
FIG. 4 depicts a schematic diagram of a fourth technique in the prior art for measuring flicker noise. The addition of the RC filter on all of the leads to the device under test enables the device under test to be powered with a conventional power supply in contrast to requiring a battery supply and is, therefore, more suitable for use in an industrial setting. Like the technique in FIG. 3, however, this technique generally provides satisfactory results for low cut-off frequency MOSFETs but not for high cut-off frequency MOSFETs or bipolar devices.
FIG. 5 depicts a schematic diagram of a fifth technique in the prior art for measuring flicker noise. In accordance with this technique, a switch selects from one of a number of load resistors, R.sub.1, R.sub.2. . . R.sub.n, in an attempt to match the load resistance to the resistance between the drain and source or the collector and emitter of the device under test. This is advantageous because it increases the sensitivity of the technique to measure flicker noise. Empirically, the presence of the switch in the circuit adds noise, which substantially offsets the gain in sensitivity that is attained from matching the load resistance to the resistance of the device under test. Like the techniques depicted in FIGS. 3 and 4, this technique generally provides satisfactory results for low cut-off frequency MOSFETs but does not for high cut-off frequency MOSFETs or bipolar devices.
Although techniques exist for measuring the flicker noise in low cut-off frequency MOSFETs in an industrial setting, there exists the need for a tool that can accurately measure in an industrial setting the flicker noise exhibited by bipolar devices and both low and high cut-off frequency MOS devices, including transistors and diodes.